Today the processing of tuna fish for canning is a largely manual operation. To prepare the fish bodies for butchering, the frozen fish are first cooked. The skin, bone, entrails, blood meat, and other unwanted portions can then be cut from the delectable loin meat by hand. Finally, the cooked loin meat, along with broth and water, is put into cans and retorted.
A number of patents and patent applications owned by The Laitram Corporation, assignee of this invention, are directed to automating the tuna canning process. For example, U.S. Pat. No. 5,367,929, "Fluid Jet Cutting Knife Apparatus," discloses a computer-controlled fluid jet knife that is capable of cutting the skin away from a fish steak whose outline has been scanned and electronically defined. Some shortcomings of the described system in removing skin are its relatively slow speed, its computational complexity, and its inability to handle indentations in the skin. U.S. Pat. No. 5,569,068, "Method and Apparatus for Skinning Fish," shows an automated scraper system that uses spring-loaded scraper plates to scrape the skin from a partly frozen fish body conveyed through the scraper field. The scraper has some advantages over the water jet system in that it does not require computer control and the scrapers generally contact the entire skin surface. Nevertheless, the scrapers tend to skip or jump over the scales and pectoral and tail fins and do not perform so well in removing skin without gouging flesh in those areas. The task is made even more difficult because of the need to keep the fish in a frozen state to facilitate downline automated process steps, such as loin separation. Thus, there is a need to improve the skinning of frozen fish.
U.S. government regulations require that the average press weight of 24 cans of tuna exceed a specified minimum. The press weight is the weight of the drained contents of the cans after undergoing a press of 384 lb/in.sup.2. The residual material after press is a combination of fish meat and retained moisture. The greater the retained weight after press, the greater is the yield because less meat must be packed into each can to meet the regulations. Because even small percentage increases in yield represent millions of dollars in income to a processor, high yield is a primary goal. Thus, another objective of the invention is to increase yield.